This invention relates to zirconia sols and to methods of making zirconia sols.
The incorporation of zirconia sols into organic matrix materials (e.g., polymers) can provide optically transparent or translucent materials having high x-ray opacity and high refractive index. The degree to which the x-ray opacity and/or refractive index of the organic matrix may be increased is a function of the loading limit of the sol in the organic matrix and the x-ray scattering capability or refractive index of the zirconia particles.
The characteristics of the zirconia sol (e.g., degree of crystallinity of the zirconia particles, crystal lattice structure, particle size and degree of primary particle association) govern the optical transmission, x-ray opacity, refractive index and the loading limit of the zirconia sol in an organic polymer. Condensed crystalline zirconia is a high refractive index material having a large x-ray scattering capability whereas amorphous hydrous zirconium salts have a lower refractive index and lower x-ray scattering capability. Optical transmission of a zirconia sol is a function of the size of the zirconia particles in the sol. As the primary particle size increases and/or the degree of association between primary particles increases the optical transmission is reduced. Loading limit of a zirconia sol in an organic matrix material is a function of both particle association and particle aspect ratio. As particle association in a zirconia sol increases, the loading limit of the zirconia sol in an organic matrix decreases. Similarly, as the aspect ratio of the zirconia particles in a sol increases, the loading limit of the zirconia particles in an organic matrix decreases. Accordingly, zirconia particles having a low aspect ratio are preferred when it is desired to incorporate high loadings of the particles in organic matrix materials. In this respect, zirconia particles having cubic and/or tetragonal crystal phases are preferred over those having a monoclinic crystal phase.
The present invention provides zirconia sols and methods for making zirconia sols wherein the sols comprise crystalline zirconia particles having a small primary particle size and substantially non-associated form. Sols of the present invention may be added to organic matrix materials (e.g., monomer, oligomers and polymers) to provide transparent or translucent zirconia filled composite materials having high index of refraction and high x-ray opacity.
In one aspect, the present invention provides zirconia sols comprising an aqueous phase having dispersed therein a plurality of single crystal zirconia particles having an average primary particle size less than about 20 nm, preferably ranging from about 7-20 nm. The zirconia sols of the present invention are substantially non associated (i.e., non aggregated and non agglomerated) having a dispersion index ranging from about 1-3, more preferably ranging from 1-2.5 and most preferably ranging from about 1-2. The zirconia sols of the present invention are highly crystalline exhibiting a crystallinity index of about 0.65 or greater, more preferably about 0.75 or greater and most preferably about 0.85 or greater. Of the crystalline phase, about 70% or greater, more preferably about 75% or greater and most preferably about 85% or greater exists in combined cubic and tetragonal crystal lattice structures.
In another aspect, the present invention provides a method of making a zirconia sol comprising the steps of:
(a) providing an aqueous solution comprising a polyether acid zirconium salt; and
(b) hydrolyzing the aqueous solution of the polyether acid zirconium salt by heating the solution at a temperature and a pressure sufficient to convert the polyether acid zirconium salt into crystalline zirconia particles.
In a preferred embodiment of the process, step (a) comprises:
(a) reacting an aqueous solution of a zirconium salt with a polyether carboxylic acid to form an aqueous solution comprising a polyether acid zirconium salt and a free acid; and
(b) optionally, removing at least a portion of the free acid.
In a preferred embodiment, the step of removing at least the free acid comprises:
(a) drying an aqueous solution of the polyether acid zirconium salt; and
(b) dispersing the dried acid polyether acid zirconium salt in water to form an aqueous solution.
Preferred zirconium salts for use as starting materials in the formation of a polyether acid zirconium salt have the general formula:
ZrO(4xe2x88x92n/2)(X)n
where X is a carboxylic acid displaceable counterion selected from the group consisting of formate, propionate, nitrate, chloride, carbonate and a combination thereof; and wherein n ranges from 0.5-4. A particularly preferred starting material is zirconium acetate.
Preferred polyether carboxylic acids for use in the process of the present invention have the general formula:
CH3xe2x80x94[Oxe2x80x94(CH2)y]xxe2x80x94X2xe2x80x94CH2)nxe2x80x94COOH
where X2 is selected from the group consisting of:
xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94C(O)Oxe2x80x94 and xe2x80x94C(O)NH;
n ranges from about 1-3;
x ranges from about 1-10; and
y ranges from about 1-4.
Examples of particularly preferred polyether carboxylic acids include 2-[2-(2-methoxyethoxy)ethoxy]acetic acid and 2-(2-methoxyethoxy) acetic acid.
In another aspect, the present invention provides a composite material comprising:
an organic matrix material having dispersed therein a plurality of single crystal zirconia particles having an average primary particle size less than about 20 nm and having a dispersion index ranging from about 1-3, wherein the zirconia particles have a crystallinity index of about 0.65 or greater and about 70% or greater combined cubic and tetragonal crystal lattice structure in the absence of an effective amount of a crystal phase stabilizer.
In a preferred embodiment, the composite material has an index of refraction of about 1.6 or greater, more preferably about 1.66 or greater and most preferably about 1.75 or greater.
In a preferred embodiment the organic matrix material is a monomer, oligomer or polymer, for example, acrylates, methacrylates, epoxies, styrenes, polyolefins, polyesters, polyurethanes, polymethylmethacrylates, polystyrenes, polycarbonates, polyimides and mixtures thereof.
As used herein, with respect to the present invention, the terms listed below shall have the following meanings.
xe2x80x9cassociated particlesxe2x80x9d as used herein refers to a grouping of two or more primary particles that are aggregated and/or agglomerated.
xe2x80x9caggregationxe2x80x9d as used herein is descriptive of a strong association between primary particles which may be chemically bound to one another. The breakdown of aggregates into smaller particles is difficult to achieve.
xe2x80x9cagglomerationxe2x80x9d as used herein is descriptive of a weak association of primary particles which may be held together by charge or polarity.
xe2x80x9cdispersion indexxe2x80x9d as used herein refers to the hydrodynamic particle size of the zirconia particles in the sol divided by the primary particle size of the zirconia particles. Theoretically, the dispersion index for non-associated particles equals 1 with the dispersion index increasing as the degree of association between primary particles increases.
xe2x80x9chydrodynamic particle sizexe2x80x9d refers to the weight average particle size of the zirconia particles in the aqueous phase as measured by Photon Correlation Spectroscopy (PCS).
xe2x80x9cprimary particle sizexe2x80x9d as used herein refers to the size of a non- associated single crystal zirconia particle.
xe2x80x9csolxe2x80x9d as used herein refers to a dispersion or suspension of colloidal particles in an aqueous phase.
xe2x80x9czirconiaxe2x80x9d as used herein refers to ZrO2 and may also be known as zirconium oxide and as zirconium dioxide.
The zirconia sols and zirconia particles of the present invention possess several advantageous characteristics. For example, the zirconia particles have a small average primary particle size and are highly crystalline. Of the crystalline portion of the zirconia particles the predominate crystal lattice structures are cubic and tetragonal with the balance being monoclinic. Cubic and tetragonal crystal lattice structures promote the formation of low aspect ratio primary particles having a cube-like shape when viewed under an electron microscope. In the sol the primary particles exist in a substantially non-associated (i.e., non aggregated and non-agglomerated) form. The particle size, crystalline nature of the particles and freedom from association of the particles allows the production of high refractive index, high x-ray opacity transparent composite materials when the sols of the present invention are incorporated into organic matrix materials, for example, monomers, oligomers and/or polymers.
Primary Particle Size:
Zirconia sols of the present invention comprise a plurality of single crystal zirconia particles having an average primary particle size of about 20 nm or less, more preferably, having an average primary particle size ranging from about 7-20 mn. As used herein, the term xe2x80x9cprimary particle sizexe2x80x9d refers to the size of a non-associated single crystal zirconia particle. Primary particle size is determined by x-ray diffraction as described in Test Procedure 3.
Crystallinity:
Zirconia sols of the present invention comprise zirconia particles which are highly crystalline in nature. This is important in that crystalline zirconia has a higher refractive index and higher x-ray scattering capability than amorphous zirconia. Crystallinity of zirconia particles may be quantified, for example, using a crystallinity index. Crystallinity index is calculated by dividing the x-ray scattering intensity of the sample material by the x-ray scattering intensity of a known crystalline standard material, for example, calcium stabilized zirconium oxide. A specific test procedure for determining the crystallinity index of zirconia particles is set forth herein in Test Procedure 4. In zirconia sols of the present invention the zirconia particles have a crystallinity index of about 0.65 or greater as measured using Test Procedure 4. More preferably, the zirconia particles having a crystallinity index of about 0.75 or greater, most preferably about 0.85 or greater as measured using Test Procedure 4.
Of the crystalline portion of the zirconia particles, the predominate crystal lattice forms are cubic and tetragonal with a minor amount of monoclinic phase also being present. Due to the difficulty in separately quantifying cubic and tetragonal crystal lattice structures using x-ray diffraction, the two have been combined and are reported herein as combined cubic and tetragonal. Specifically, the zirconia particles comprise about 70% or greater combined cubic and tetragonal crystal lattice structure. More preferably, the zirconia particles comprise about 75% or greater combined cubic and tetragonal crystal lattice structure, and most preferably comprise about 85% or greater combined cubic and tetragonal crystal lattice structure. In each instance, the balance of the crystalline phase is in the monoclinic crystal lattice structure.
Due to their very small size, the zirconia particles exist in predominately cubic and tetragonal crystal lattice phases without need for an effective amount of a crystal phase stabilizer. As used herein the term xe2x80x9ccrystal phase stabilizerxe2x80x9d refers to a material which may be added to stabilize zirconia in the cubic and/or tetragonal crystal lattice structure. Specifically, crystal phase stabilizers function to suppress transformation from the cubic and/or tetragonal phase to the monoclinic phase. Crystal phase stabilizers include, for example, alkaline-earth oxides such as MgO and CaO, rare earth oxides (i.e., lanthanides) and Y2O3. As used herein the term xe2x80x9can effective amountxe2x80x9d refers to the amount of crystal phase stabilizer necessary to suppress transformation of zirconia from the cubic and/or tetragonal phase to the monoclinic phase. In a preferred embodiment, the zirconia particles comprise less than about 1 wt. % of a crystal phase stabilizer, more preferably less than about 0.1 wt. % of a crystal phase stabilizer.
Dispersion Index:
In zirconia sols of the present invention, the primary particles of zirconia exist in a substantially non-associated (i.e., non-aggregated and non-agglomerated) form. A quantitative measure of the degree of association between the primary particles in the sol is the dispersion index. As used herein the xe2x80x9cdispersion indexxe2x80x9d is defined as the hydrodynamic particle size divided by the primary particle size. The primary particle size is determined using x-ray diffraction techniques as described in Test Procedure 3. Hydrodynamic particle size refers to the weight average particle size of the zirconia particles in the aqueous phase as measured by Photon Correlation Spectroscopy (PCS) (see, Test Procedure 5). If the primary particles are associated, PCS provides a measure of the size of the aggregates and/or agglomerates of primary particles in the zirconia sol. If the particles are non-associated, PCS provides a measure of the size of the primary particles. Accordingly, as the association between primary particles in the sol decreases the dispersion index approaches a value of 1. In zirconia sols of the present invention the primary zirconia particles exist in a substantially non-associated form resulting in a zirconia sol having a dispersion index ranging from about 1-3, more preferably ranging from about 1-2.5, and most preferably ranging from about 1-2.
Optical Transmission:
Zirconia sols of the present invention may be characterized in part as having a high optical transmission due to the small size and non-associated form of the primary zirconia particles in the sol. High optical transmission of the sol is an important characteristic in preparing transparent or translucent zirconia-filled composite materials. As used herein, xe2x80x9coptical transmissionxe2x80x9d refers to the amount of light that passes through a sample (e.g., a zirconia sol of the present invention) divided by the total amount of light incident upon the sample and may be calculated using the following equation:
%Transmission=(I/IO)
where: I is the light intensity passing though the sample; and
IO is the light intensity incident on the sample.
Optical transmission may be determined using an ultraviolet/visible spectrophotometer such as that commercially available as Model 6-550 Pye Unicam (from Pye Unicam Ltd., Cambridge England).
For zirconia sols of the present invention having a percent zirconia of about 1.28 wt. %, the optical transmission is preferably about 70% or greater, more preferably about 80% or greater, and most preferably about 90% or greater when tested in accordance with Test Procedure 2. For zirconia sols of the present invention having a percent zirconia of about 10 wt. %, the optical transmission is preferably about 20% or greater, more preferably about 50% or greater, and most preferably about 70% or greater when tested in accordance with Test Procedure 2.
Method of Making Zirconia Sols:
Zirconia Precursor:
Suitable starting materials for preparing polyether acid zirconium salts include basic zirconium salts such as zirconium carboxylates and basic zirconium salts having counterions that may be displaced with carboxylic acids. Representative examples of basic zirconium salts having counterions that may be displaced with carboxylic acids include zirconium oxynitrate, zirconium oxychloride and zirconium carbonates. Basic zirconium salts are salts of zirconium wherein at least a portion of the cationic charge on the zirconium is compensated by hydroxide or an O2xe2x88x92 anion. Because it is difficult in practice to determine whether the oxygen content in basic zirconium salts arises from bound hydroxide or O2xe2x88x92, it is common to represent this oxygen content as simply oxygen. Thus, formula (1) set forth below is presented with bound water excluded for simplicity and represents a general formula for zirconium compounds that may be suitable as starting materials for preparing polyether acid zirconium salts.
ZrO(4xe2x88x92n/2)(X)nxe2x80x83xe2x80x83(1)
where: X is a carboxylic acid displaceable counterion; and
n ranges from 0.5 to 4.
Representative examples of carboxylic acid displaceable counterions include carboxylates such as acetates, formates and propionates and other counterions such as nitrate, chloride, carbonate or a combination thereof. Zirconium alkoxides, although not formally zirconium salts, may be used as starting materials in the formation of the polyether acid zirconium after initial reaction with a suitable acid to form a basic zirconium salt.
A preferred starting material is an aqueous solution or sol of basic zirconium acetate having the general formula ZrO(4xe2x88x92n/2)(CH3COO)n. where n ranges from about 1-2. In aqueous solutions, zirconium acetate probably exists as complex polynuclear zirconium cation. Processes for making zirconium acetate are well known in the art (see, for example, W. B. Blumenthal, xe2x80x9cThe Chemical Behavior of Zirconiumxe2x80x9d, D. Van Nostrand Company, Princeton, N.J., pp. 311-338). Suitable zirconium acetate solutions comprise from about 5-40 wt. % as ZrO2 and range from about 5-40 wt. % acetate. A preferred zirconium acetate sol starting material comprises ZrO1.25(C2H3O)1.5 at 20 wt. % ZrO2 and is commercially available under the trade designation xe2x80x9cNyacol ZrO2(Ac)xe2x80x9d from Nyacol Products Corporation, Ashland, Mass.
Polyether Carboxylic Acid:
In a preferred process of the present invention a polyether acid zirconium salt is prepared by reacting, in an aqueous solution, a zirconium salt with a polyether carboxylic acid. As presently understood, the polyether carboxylic acid is believed to function to prevent association (i.e., agglomeration and/or aggregation) of the zirconia particles as they are formed during the hydrolysis reaction. In this way, the zirconia particles produced according to the process of the present invention are substantially non-associated.
Polyether carboxylic acids suitable for use as modifiers in the present invention are water soluble monocarboxylic acids (i.e., containing one carboxylic acid group per molecule) having a polyether tail. The polyether tail comprises repeating difunctional alkoxy radicals having the general formula xe2x80x94Oxe2x80x94Rxe2x80x94. Preferred R groups have the general formula xe2x80x94CnH2nxe2x80x94 and include, for example, methylene, ethylene and propylene (including n-propylene and i-propylene) or a combination thereof. Combinations of R groups may be provided, for example, as random, or block type copolymers.
A preferred class of monovalent polyether radicals may be represented generally by formula (3):
CH3xe2x80x94[Oxe2x80x94(CH2)y]xxe2x80x94Xxe2x80x94COOHxe2x80x83xe2x80x83(3)
where:
X is a divalent organic linking group;
x ranges from about 1-10; and
y ranges from about 1-4.
Representative examples of X include xe2x80x94X2xe2x80x94(CH2)nxe2x80x94 where X2 is xe2x80x94Oxe2x80x94 xe2x80x94Sxe2x80x94, xe2x80x94C(O)Oxe2x80x94, xe2x80x94C(O)NHxe2x80x94 and wherein n ranges from about 1-3.
Examples of preferred polyether carboxylic acids include 2-[2-(2-methoxyethoxy)ethoxy] acetic acid having the chemical structure CH3O(CH2CH2O)2CH2COOH (hereafter MEEAA) and 2-(2-methoxyethoxy) acetic acid having the chemical structure CH3OCH2CH2OCH2COOH (hereafter MEAA). MEAA and MEEAA are commercially from Aldrich Chemical Co., Milwaukee, Wis. as catalog numbers 40,701-1 and 40,700-3, respectively. It is also within the scope of this invention to utilize a mixture of more than one polyether carboxylic acid.
Reaction of the polyether carboxylic acid with a zirconium salt following reaction sequence (1):
Zro(4xe2x88x92n/2)(X)n+a R2xe2x80x94COOHxe2x86x92ZrO(4xe2x88x92n/2)(X)nxe2x88x92a(R2COO)a+a HXxe2x80x83xe2x80x83(1)
results in the formation of a polyether acid zirconium salt having the general formula ZrO(4xe2x88x92n/2)(X)nxe2x88x92a(R2COO)a and liberates (i.e., releases) approximately a stochiometric amount of an acid having the general formula HX. By way of example, when the zirconium salt comprises zirconium acetate (ZrO(4xe2x88x92n/2)(C2H3O2)n) a near stochiometric amount of acetic acid (C2H3O2H) is released as a result of the formation of the polyether acid zirconium salt (see, reaction sequence 1a).
ZrO(4xe2x88x92n/2)(C2H3O2)n+a R2xe2x80x94COOHxe2x86x92ZrO(4xe2x88x92n/2)(C2H3O2)nxe2x88x92a(R2COO)a+a C2H3O2Hxe2x80x83xe2x80x83(1a)
Salts of zirconium with carboxylic acids are not dissociated in the aqueous phase as the acid is bound to the zirconium atom. The carboxylic acid effects the water solubility of the salt. Attachment of hydrophobic acids (e.g., alkyl acids) to the zirconium causes the salts to be insoluble in water. In fact, even the addition of small acids such as propionic acid and acrylic acid cause the salt to be insoluble in water. In contrast, the polyether acids used in the present invention allow higher molecular weight acids to be used while maintaining the water solubility of the polyether acid zirconium salt. This in turn allows hydrothermal treatment of the dissolved polyether acid zirconium salt in the aqueous phase.
Typically, relative to the zirconium salt starting material, the polyether carboxylic acid is added in an amount ranging from about 2.5-5.0 millimoles per gram equivalent of ZrO2 in the zirconium salt. For the preferred zirconium acetate starting material (i.e., Nyacol ZrO2(Ac)), this range results in the displacement of about 20-50% of the acetate groups. Preferably, the amount of polyether carboxylic acid added should be limited to the minimum amount necessary to prevent association of the resulting zirconia particles. In this way, the amount of acid released during formation of the polyether acid zirconium salt is kept to a minimum. The amount of polyether carboxylic acid added may depend upon such factors as, for example, the molecular weight of the polyether carboxylic acid, the concentration, time and temperature during the hydrolysis reaction.
Typically, the polyether carboxylic acid is added to an aqueous solution of the zirconium salt and the resulting solution is stirred at room temperature for about 30-60 minutes. The polyether carboxylic acid molecules react with the zirconium salt displacing and substituting for at least a portion of the acid groups bound to the zirconium salt. The displaced acid groups are released into the solution as free acid. It will ordinarily be preferred to remove at least a portion of the acid, more preferably substantially all of the acid released during the formation of the polyether acid zirconium salt. It should be noted that removal of the acid may function to shift the reaction equilibrium towards formation of the polyether acid zirconium salt. Suitable techniques for removing the excess acid are known in the art and include, for example, drying or distillation. When the liberated acid has a low boiling point (e.g.,  less than about 175xc2x0 C.), it may be removed by heating the solution until the aqueous phase evaporates leaving a residue of the polyether acid zirconium salt. The polyether acid zirconium salt must then be dissolved in water prior to hydrolysis.
Hydrolysis:
After formation of the polyether acid zirconium salt and, preferably, removal of the liberated acid, the next step is to hydrolyze an aqueous solution of the polyether acid zirconium salt under conditions sufficient to convert the polyether acid zirconium salt into crystalline zirconia particles. By way of example, when the polyether acid zirconium salt is derived from the acetate salt (see, reaction sequence 1a), the hydrolysis step follows general reaction sequence (2a):
ZrO(4xe2x88x92n/2)(C2H3O2)nxe2x88x92a(R2COO)axe2x86x92acid modified ZrO2+(nxe2x88x92a) C2H3O2H+a R2COOHxe2x80x83xe2x80x83(2a)
The hydrolysis reaction forms acid modified zirconia particles and also produces free carboxylic acids (i.e., C2H3O2H and R2COOH) as a by product. Therefore, the resultant zirconia sol comprises the acid modified zirconia particles and a mixture of two carboxylic acids in water. By acid modified zirconia particles it is meant that at least a fraction of the acids are adsorbed to the surface of the zirconia particles.
The hydrolysis reaction of the polyether acid zirconium salt solution may take place in any suitable reaction vessel. Since the reaction is typically performed under high temperatures and pressures, an autoclave will generally be the preferred type of reaction vessel. One example of a preferred reaction vessel is commercially available as Pressure Reactor Series #4520xe2x80x9d from Parr Instruments Co., Moline, Ill.
In operation, an aqueous solution of the polyether acid zirconium salt is first charged into a reaction vessel. The concentration of the polyether acid zirconium salt solution is typically in the range of 0.5-3 wt. % ZrO2, preferably in the range of 1-2 wt. % ZrO2. However, the concentration may be varied through a wider range depending upon the other reaction conditions. The polyether acid zirconium salt solution is then heated to a temperature sufficient to convert it into zirconia particles. Preferred hydrolysis temperatures range from about 140-250xc2x0 C., more preferably ranging from about 150-200xc2x0 C. Typically the reaction vessel is heated to the desired hydrolysis temperature over a period of several hours. Among other considerations, a suitable hydrolysis temperature or temperature range, may be selected in order to minimize degradation and/or decomposition of the polyether carboxylic acid. The pressure maintained in the reaction vessel may be the autogenous pressure (i.e., the vapor pressure of water at the temperature of the reaction) or, preferably, the reaction vessel may be pressured, for example, with an inert gas such as nitrogen. Preferred pressures range from about 1-30 bars, more preferably 2-20 bars. Pressurization of the reaction vessel is believed to reduce or eliminate refluxing of the polyether acid zirconium salt solution within the reaction vessel which may deleteriously affect the properties of the resulting zirconia sol. The time of hydrolysis is typically a function of the hydrolysis temperature and the concentration of the salt solution. Heat is typically applied until the hydrolysis reaction is substantially complete. Generally, the time involved is in the range of about 16-24 hours at a temperature of about 175xc2x0 C., however, longer or shorter times may also be suitable. The reaction may be monitored by examining the resulting zirconia particles using x-ray diffraction or by examining the amount of free acid in the water phase using IR spectroscopy or HPLC. Upon completion of the hydrolysis, the pressure vessel is allowed to cool and the resulting zirconia sol is removed from the reaction vessel. Although the procedure described above is a batchwise process, it is also within the scope of this invention to conduct the hydrolysis in a continuous process.
Post-Treatment of Zirconia Sols:
Zirconia sols of the present invention may be concentrated by removing at least a portion of the liquid phase using techniques well known in the art, for example, evaporation or ultra-filtration. In a preferred method the zirconia sols are concentrated to about 10-40 wt. % ZrO2 using a rotary evaporator.
Zirconia sols prepared in accordance with the method of the present invention typically contain an excess of acid over that normally desired (see, reaction sequence 2a). When it is desired to combine a zirconia sol of the present invention with an organic matrix material, for example, an organic monomer, it will ordinarily be necessary to remove at least a portion of, more preferably substantially all of, the free acid present in the sol. Typically, the acid may be removed by such conventional methods as drying, dialysis, precipitation, ion exchange, distillation or diafiltration.
Due to the formation of free acid during the hydrolysis reaction, the pH of the as prepared zirconia sols typically ranges from about 1.8-2.2. Dialysis may be used to increase the pH of the sols. Dialyzed sols typically have a pH ranging about 1-4.5, or greater, depending upon the extent of the dialysis. The pH of the sols may also be adjusted by the addition of acids (e.g., concentrated HCl and glacial acetic) and/or base (e.g., aqueous ammonia). Addition of aqueous ammonia has resulted in clear sol to at least pH 6-7.
Dialysis, ion exchange and diafiltration methods may be used to remove the free acid without substantially changing the ratio of the acids adsorbed to the surface of the zirconia particles. Alternatively, removal of excess acid and concentration of the sol may be achieved by first evaporating the water and free acid from the sol to obtain a dry powder. The dry powder may then be redispersed in a desired amount of water to obtain a concentrated sol substantially free of excess acid. It should be noted, however, that this technique may change the ratio of the acids adsorbed to the surface of the zirconia particles in such a way that the ratio of the higher boiling acid to the lower boiling acid is increased.
Optionally, after formation of the zirconia sol, the polyether carboxylic acid groups may be removed or displaced from the zirconia particles of the sol. Removal of the polyether carboxylic acid groups may be advantageous, for example, when the polyether groups would be incompatible with an organic matrix material to which it is desired to add the zirconium sol. Displacement of the polyether carboxylic acid groups may be accomplished, for example, by displacing the polyether acid from the zirconia particles with a carboxylic acid, for example, acetic acid. The carboxylic acid displaces and substitutes for the polyether carboxylic acid groups on the zirconia particles. After displacement, the free polyether carboxylic acid may be removed from the sol using techniques known in the art, for example, dialysis or diafiltration.
Surface Modification:
In some instance it may be desirable to combine a zirconia sol of the present invention with an organic matrix material, for example a monomer, oligomer and/or polymer. The zirconia particles may be added to a organic matrix materials to provide matrix materials having increased index of refraction and increased radiopacity. Specifically, the zirconia particles may provide increased index of refraction and/or increased radiopacity without detrimentally affecting the optical transmission of the organic matrix.
Generally it will be necessary to surface modify the zirconia particles in order to provide compatibility with an organic matrix material. Surface modification involves reacting the zirconia particles with a surface modification agent or combination of surface modification agents that attach to the surface of the zirconia particles and which modify the surface characteristics of the zirconia particles to provide increased compatibility with the organic matrix material.
Surface modification agents may be represented by the formula A-B where the A group is capable of attaching to the surface of a zirconia particle, and where B is a compatibilizing group which may be reactive or non-reactive with the organic matrix. Groups capable of attaching, via adsorption, to the surface of a zirconia particle include, for example, acids such as carboxylic acids, sulfonic acids, phosphonic acids and the like. Compatibilizing groups B which impart polar character to the zirconia particles include, for example, polyethers.
Representative examples of polar modifying agents having carboxylic acid functionality include MEEAA, MEAA and mono(polyethylene glycol)succinate. Compatibilizing groups B which impart non-polar character to the zirconia particles include, for example, linear or branched aromatic or aliphatic hydrocarbons. Representative examples of non-polar modifying agents having carboxylic acid functionality include octanoic acid, dodecanoic acid and oleic acid. Modifying agents reactive with the organic matrix include, for example, acrylic acid, methacrylic acid and mono-2-(methacryloxyethyl)succinate. A useful surface modification agent which imparts both polar character and reactivity to the zirconia particles is mono(methacryloxypolyethyleneglycol) succinate. This material may be particularly suitable for addition to radiation curable acrylate and/or methacrylate organic matrix materials.
Generally, the surface modification may be accomplished by simple addition of a surface modifying agent to a zirconia sol of the present invention. Optionally, a water miscible cosolvent may be used to increase the solubility of the surface modifying agent and/or compatibility of the surface modified particles in the aqueous phase. Suitable cosolvents include water-miscible organic compounds, for example, methoxy-2-propanol or N-methyl pyrrolidone. When the surface modification agents are acids, the modification of the zirconia particles typically does not require elevated temperatures.
Various methods may be employed to combine the zirconia sol of the present invention with an organic matrix material. In one aspect, a solvent exchange procedure may be utilized. In the solvent exchange procedure the organic matrix material is first added to the surface modified sol. Optionally, prior to addition of the organic matrix material, a cosolvent such as methoxy-2-propanol or N-methyl pyrolidone may be added to the zirconia sol to help miscibilize the organic matrix material in the water. After addition of the organic matrix material, the water and cosolvent (if used) are removed via evaporation, thus leaving the zirconia particles dispersed in the organic matrix material. The evaporation step may be accomplished for example, via distillation, rotary evaporation or oven drying.
Alternatively, another method for incorporating a zirconia sol of the present invention into an organic matrix material involves drying of the zirconia particles to produce a powder followed by the addition of the organic matrix material into which the particles are dispersed. The drying step may be accomplished by conventional means such as oven drying or spray drying. In another aspect, conventional oven drying can be performed at between about 70xc2x0 C. to 90xc2x0 C. for about 2 to 4 hours.
Alternatively, another method of incorporating a zirconia sol of the present invention into an organic matrix material involves first surface treating the zirconia particles with a non-polar carboxylic acid, for example, oleic acid. The non-polar acid surface modifies the zirconia particles causing them to flock into a filterable mass. The particles may then be separated from the liquid phase via filtration, optionally dried, and combined with the organic matrix material.
In yet another method the surface modified particles can be extracted into a water immiscible solvent or monomer, for example, toluene, hexane, ethyl acetate or styrene.
The sols of the present invention may be combined with organic matrix materials, for example, monomers, oligomers and polymers by the various techniques discussed above. The resultant composite material can have the properties of optical clarity, high refractive index and high radiopacity combined with high modulus, hardness, and the processibility and flexibility of the polymer matrix. Suitable materials for incorporated zirconia sols of the present invention include, for example, dental materials as described in U.S. Ser. No. 09/428,937 xe2x80x9cDental Materials With Nano-Sized Silica Particlesxe2x80x9d (filed on Oct. 28, 1999) and U.S. Ser. No. 09/428,185 xe2x80x9cRadiopaque Dental Materials With Nano-Sized Particlesxe2x80x9d (filed on Oct. 28, 1999), the disclosures of which are incorporated herein by reference. In general, the refractive index of a composite material increases linearly with volume fraction of the zirconia particles in the organic matrix. To obtain a high index of refraction, an organic matrix material having a high index of refraction is generally preferred. Zirconia particles from the zirconia sol of the present invention may be used to further increase the refractive index of the organic matrix. When combined with an organic matrix material the resulting composite materials may achieve a refractive index of about 1.6 or greater, more preferably about 1.66 or greater and most preferably about 1.75 or greater.
Representative examples of polymerizable monomers include acrylates, methacrylates, styrenes, epoxies and the like. Also, reactive oligomers such as acrylated or methacrylated polyesters, polyurethanes or acrylics may also be used. The resulting composite material may be shaped or coated and then polymerized, for example, via a free-radical photopolymerization mechanism. Photopolymerization may be initiated by the use of a photoinitiator such as that commercially available under the trade designation xe2x80x9cIRGACURE 184xe2x80x9d (Ciba Specialty Chemicals, Tarrytown, N.Y.). The sols of the present invention may also be combined with other types of polymers, for example, polyolefins, polyesters, polyurethanes, polymethylmethacrylates, polystyrenes, polycarbonates and polyimides. Suitable techniques for combining the sol with a thermoplastic polymer include, for example, extrusion, milling or brabender mixing. Surface modification agents should be selected to be stable at the desired processing temperature.